A variety of radio apparatuses have been downsized recently. Cellular phones, a typical example of the radio apparatuses, now offer services not only a voice communication but also a data communication such as transmitting and receiving text or video. In the foregoing market situation, performance of antenna devices which transmit and receive radio-wave is one of critical factors determining the specification of the radio apparatuses. The market requires cellular phones to transmit/receive radio-waves of plural frequency-bands sensitively with a single antenna device.
A cellular phones having a conventional antenna device is described with reference to FIGS. 12 and 13. FIG. 12 shows a schematic diagram illustrating a cellular phone to which a conventional antenna device is mounted. FIG. 13 shows a perspective view illustrating an appearance of the conventional antenna device.
As shown in FIG. 12, antenna device 3 to be mounted to a portable radio apparatus is placed in parallel with ground plane 8. Antenna device 3 includes first antenna element 1 (hereinafter called “antenna 1”) for resonating with a first frequency and second antenna element 2 (hereinafter called “antenna 2”) for resonating with a second frequency. Antenna device 3 is coupled to feeding point 4 placed on ground plane 8, and further coupled to radio circuit 7 via matching circuit 5 and transmission line 6. Radio apparatus 9 is formed of the elements including from antenna device 3 to radio circuit 7 and also ground plane 8.
As shown in FIG. 13, antenna device 13 includes base-section 11 made of electrically insulating resin and shaped like a rectangular parallelepiped. Antenna 1 and meander-shaped antenna 2 are fixed to base-section 11. Antenna 1 has helical coil-section 1A, and antenna 2 is insulated from antenna 1. Both antenna 1 and antenna 2 are commonly fed by feeding point 4.
For the description to proceed, assume that antenna 1 of antenna device 3 thus formed resonates with the frequency band of Global System for Mobile Communications (GSM: 880–960 MHz) and antenna 2 resonates with the frequency band of Digital Communication System (DCS: 1710–1880 MHz).
Reception of a GSM radio-wave at antenna 1 excites a current, which runs through feeding point 4, matching circuit 5, and transmission line 6, then arrives at radio circuit 7, whereby the GSM radio-wave is received.
When a GSM radio-wave is to be transmitted, a signal generated at radio circuit 7 runs through transmission line 6, matching circuit 5, feeding point 4, and arrives at antenna 1, which then excites the signal for radiating. The GSM radio-wave is thus transmitted.
In the case of DCS, antenna 2 receives/transmits the DCS radio-wave via feeding point 4 in the same manner as antenna 1 does.
Conventional antenna device 3 is, e.g. disclosed in Japanese Patent Unexamined Publication No. 2003-101335.